Fuel cells have been used as a power source in many applications. In the “pure hydrogen” type of proton exchange membrane (PEM) fuel cells, a hydrogen reactant (i.e., a reactant having a hydrogen concentration of approximately 80% by volume or greater) is supplied to the anode sides of the fuel cell, and oxygen is supplied as the oxidant to the cathode sides. Each cell within the stack includes a membrane electrode assembly (MEA) which provides its increment of voltage. MEAs include a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and a cathode catalyst on the opposite face. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack.
In practice, the reactants are supplied to the fuel cells through individual inlet manifolds and headers. In the inlet header the reactant, for instance the anode flow, is divided in a number of flow paths feeding individual cells. The exhaust flow leaves the cells, mixes in an outlet header and exits the stack through an outlet manifold. A coolant is also provided to each segment to remove heat generated by the reduction of reactants. In at least one known design, the anode sides of all cells are fed in parallel, i.e., they have the same inlet hydrogen concentration.
A disadvantage of parallel feeding a single group of cells is that the fuel cell stack is unable to stably operate at low stoichiometry; that is, near the mass flow of reactants needed to satisfy a given power output. It is therefore difficult to achieve efficient hydrogen or oxygen utilization. As a result, system efficiency is not optimized.
Stack designs which partially correct the above situation are known, such as the stack design of U.S. Pat. No. 5,478,662 issued to Strasser. In stacks such as the Strasser design, individual groups of parallel cells are arranged wherein the flow within each cell of each group is in parallel, and all the flow from each group flows between groups in series. The number of individual fuel cells normally varies in these stack designs wherein the initial or upstream segments of cells contain the largest number of individual fuel cells and each successive segment provides a reduced quantity of fuel cells. With this type of configuration the last segment of the set of segments normally has the fewest number of individual fuel cells.
The above series/parallel stack designs normally provide a serpentine type flow pattern throughout the stack. A serpentine flow path results in both anode and cathode side reactant flows which are either horizontal throughout the stack, or that must overcome gravity for one or more individual segments. Water build-up in the lower portions of the stack inhibits reactant contact with the catalyst materials of the fuel cells, thus decreasing stack efficiency.
A further drawback of known fuel cell stack designs is the inability to control coolant supply and coolant supply locations. Common hydrogen reactant fuel cell stacks do not provide for stack humidity control by directing coolant to specific locations of the stack.